1. Field of Invention
This invention relates to precision polishing apparatuses, in particular to an improved shaft system for more uniform polishing at lower cost.
2. Prior Art
As science and technology advance, the requirements of flatness and parallelism for surfaces of certain components become more and more precise. High precision surfaces are required in the manufacture of semiconductors, electro-optic equipment, and optical instruments, as well as many other scientific applications. Consequently, many polishing machines and polishing apparatuses have been developed for improving the flatness and parallelism of polished surfaces.
As shown in FIG. 1, a conventional polishing machine includes an annular polishing pad 12 which is mounted on a polishing plate 11 and rotated in the horizontal around a vertical axis passing through the center of pad 12. An object 13 to be polished is fixedly but releasably mounted on a carrier 14. Carrier 14 with object 13 is placed on polishing pad 12. The upper surface of polishing pad 12 is the polishing surface and the bottom surface of object 13 is the surface to be polished. In order to protect the surface to be polished, a conditioning ring 15 is used around object 13. Flat and parallel surfaces can only be achieved if there is uniform removal of stock from the surface to be polished. For this purpose, the surface to be polished should move downwards without yaw in the course of polishing. Namely, the surface to be polished should always remain parallel to its previous position during the process of polishing.
Many methods and apparatuses have been developed for the purpose of keeping the surface to be polished in a series of parallel planes in the course of polishing. One of the developed methods involves connecting the carrier and the conditioning ring by a sheet of special material which is stiff in its own plane but flexible in the direction perpendicular to the sheet plane, as described in the U.S. Pat. No. 5,716,258 of Robert L. Metcalf, 1998. In that case, the object to be polished will be moved with the conditioning ring on the polishing pad as a single unit, but the object to be polished can be freely moved up and down relative to the conditioning ring as needed. The shortcoming of this method is that the connection is neither strong nor precise enough.
Another method of connecting the carrier and the conditioning ring is by means of a cylindrical shaft system. As shown in FIG. 2, a shaft 17 is fixed on carrier 14, and a bushing 16 is fixed on a conditioning ring with top 15A. Shaft 17 can be freely moved along its axis within bushing 16. Bushing 16 and shaft 17 are used to control the movement of object 13. Object 13 assembled together with carrier 14 and shaft 17 as a single unit is rotated with conditioning ring 15A synchronously on the polishing surface. At the same time, object 13 can also move down freely relative to conditioning ring 15A as the stock is removed from the surface to be polished. If the object to be polished moves up and down without yaw, flat and parallel surfaces will be obtained. Two English companies (Logitech Ltd. and Lapmaster International Ltd.) manufacture this type of polishing apparatus. The problem with this connecting method by a shaft system is that it is very difficult to manufacture such a precision cylindrical surface. While the shaft moves along its axis within the bushing, possible lateral movement (yaw) is caused by the clearance between the shaft and the bushing. Supposing the connecting length between the shaft and the bushing in direction of axis is 55 mm, a clearance of 0.5 xcexcm may cause 2xe2x80x3 (angular amount) of yaw in the movement of the shaft. This 0.5 xcexcm clearance is controlled by the dimensional tolerance and geometric tolerance of the shaft and the bushing. The yaw will translate to the surface to be polished as errors of flatness and parallelism. Therefore, the amount of yaw is controlled by the dimensional and geometric tolerance of the shaft and the bushing. Even with the use of expensive ultra-precision equipment, it is not easy to manufacture and verify, with sub-micron accuracy, a cylindrical surface. Thus, the accuracy of polishing apparatuses with cylindrical shaft systems is limited. What is more, the dimensional tolerance and geometric tolerance must be satisfied simultaneously during manufacture, or the shaft and/or bushing will not be usable. Even a highly skilled machinist has trouble consistently producing high quality cylindrical shafts. For the same reasons, once the shaft or/and the bushing becomes worn, it is difficult to repair and the worn shaft system usually is simply discarded. Consequently, the costs involved with the production and use of precision polishing apparatuses with cylindrical shaft systems are very high.
In this invention, a plane constructed shaft system is used to replace the cylindrical shaft system in the precision polishing apparatus. The advantages of the plane constructed shaft system are:
1. Since the cylindrical surface is replaced by plane surfaces in the improved shaft system, only general equipment such as lapping/polishing machinery and optical flats are needed to manufacture the plane constructed shaft system, and therefore even a machinist with only general skills can do this job.
2. The clearance between the shaft and the bushing can be adjusted or reduced to a very small amount by a special manufacturing/assembling method in which we don""t have to consider the dimensional tolerance, but just pay attention to the geometric tolerance. Consequently, the accuracy of the plane constructed shaft system will be more precise than that of the cylindrical shaft system.
3. No parts in this plane shaft system need be discarded because of over polishing in the manufacturing. The productivity of this plane shaft system will be much higher than that of the cylindrical shaft system.
4. The parts in this plane shaft system can be repaired after they wear out. Consequently, the lifetime of this plane shaft system will be prolonged indefinitely.
5. The flat parts of this plane shaft system can be polished by another polishing apparatus of the plane shaft type, and this will yield better flatness and parallelism. Thus the next generation polishing apparatus will be even better, making the manufacturing process a spirit up cycle.
6. A polishing apparatus equipped with a plane constructed shaft system will be more precise and lower in cost. Furthermore, a precision polishing apparatus equipped with a polishing pressure detecting device and a stock removal controlling device will be more efficient.
Further objects and advantages of this invention are illustrated in the ensuing drawings and descriptions.
These and other objects and features of the invention are described in the disclosure, which includes the above and ongoing written specifications, along with the claims and the drawings.